qt5.qtimageformats: unvendor libwebp

It was most likely affected by CVE-2023-4863.

This is a minimalist version of 72e396285b (from nixpkgs master),
as I don't think we need to add extra functionality/formats on stable.

pkgs/development/libraries/qt-5/modules/qtimageformats.nix

@@ -1,7 +1,7 @@
-{ qtModule, qtbase, libtiff }:
+{ qtModule, qtbase, libtiff, libwebp }:
 
 qtModule {
   pname = "qtimageformats";
   qtInputs = [ qtbase ];
-  propagatedBuildInputs = [ libtiff ];
+  propagatedBuildInputs = [ libtiff libwebp ];
 }